Crosslinked polyethylene derived from low density polyethylene is widely used as an insulating material of a CV cable (crosslinked polyethylene-insulated vinyl jacket power cable) because of its excellent electric characteristics and heat resistance. However, since it meets dielectric breakdown at ultra-high voltages, it has been demanded to develop insulating materials having higher performance characteristics.
Various attempts have been made to improve the dielectric breakdown characteristics at ultra-high voltages as described, e.g., in T. Fukada et al., I.E.E.E., E117 (5), (1982).
If there are voids or impurities, such as water, metals, etc., electric charges are concentrated thereto, thereby causing reduction in dielectric breakdown characteristics. For this reason, studies have been directed mainly to the techniques of removing such impurities. For example, clean polyethylene containing no impurities of 100 .mu.m or more in particle size or a dry crosslinking method which does not produce voids has been proposed as disclosed, e.g., in M. Takaoka, I.E.E.E., Trans. Power Appar. Syst., 102 (9), 3254-3263 (1983). A practical application of these techniques has led to production of a 275 KV cable. However, crosslinked polyethylene-insulated power cables for such a high voltage have a disadvantage of a seriously increased thickness of the insulating layer. It has been, therefore, desired for the cable insulating material to have further improved dielectric characteristics.
Another attempt is to add a voltage stabilizer, such as calcium stearate, polystyrene, various aromatic compounds, etc., considering that the thickness of the insulating layer can be decreased if the insulating material per se functions to prevent concentration of electric charges, as described, e.g., in Japanese Patent Publication No. 24809/73, West German Pat. No. 1,248,773, French Pat. No. 1,464,601, etc. The addition of such an additive, however, suffers from a disadvantage in that the additive bleeds out and the performances of the cable cannot be maintained over a long period of time.
Known comonomers that are copolymerizable with ethylene and relevant to the comonomer component according to the present invention include 2-hydroxy-4-(3-methacryloyloxy-2-hydroxypropyloxy)benzophenone described in U.S. Pat. No. 3,162,676, 2-(hydroxyphenyl)-5-acryloylaminobenzotriazole described in U.S. Pat. No. 3,072,585, 2-(2-hydroxy-4-methyl-3-propenylphenyl)benzotriazole described in British Pat. No. 981,539, etc.
In practice, however, these known comonomers often encounter various problems hard to solve, such as high prices, low purity, poor copolymerizability, difficulty in increasing a degree of polymerization, and the like. Therefore, this methodology has not been placed into practical use, still awaiting further investigations.
In the particular cases of producing ethylene copolymers by high-pressure polymerization in which it is keenly demanded not to involve a high cost and the production conditions are strictly limited, it is considered virtually impossible to industrialize copolymers using these monomers.
In some detail, it is industrially difficult to increase the purity of the above-described 2-hydroxy-4-(3-methacryloyloxy-2-hydroxypropyloxy)benzophenone that is relevant to the vinyl monomer of the formula (II) hereinafter described according to the present invention to 70% or higher due to restrictions on the process for production. The impurities adverseLy influence polymerizability and performance properties of the resulting copolymers, resulting in a failure to solve the above-described problems.
In addition, although the above-described monomer possibly promises to be copolymerized with ethylene, it finds a great technical difficulty in copolymerizing with ethylene unlike methyl acrylate, styrene, etc. which are relatively easy to copolymerize with ethylene. Therefore, this monomer has not yet been turn to practical use.